1. Field of the Invention
The teachings herein relate to formation evaluation tools used in subterranean exploration, and in particular, to devices and techniques for measuring properties of geologic formations.
2. Description of the Related Art
Various tools are used in hydrocarbon exploration and production to measure properties of geologic formations during or shortly after the excavation of a borehole. The properties are measured by formation evaluation tools and other suitable devices, which are typically integrated into a bottomhole assembly.
Such tools provide for the measurement versus depth and/or time of one or more physical quantities in or around a borehole. The taking of these measurements may be referred to as “logging”, and a record of such measurements may be referred to as a “log”.
Examples of logging processes include measurement-while-drilling (MWD) and logging-while-drilling (LWD) processes, during which measurements of properties of the formations and/or the borehole are taken downhole during or shortly after drilling. The data retrieved during these processes may be transmitted to the surface, and may also be stored with the downhole tool for later retrieval.
The tools used in MWD and LWD processes are typically included as part of a bottomhole assembly located at the lower portion of a drillstring, and may include various sensors or transducers for continuously or intermittently measuring properties of the formations and/or borehole.
LWD formation evaluation tools use real-time clocks that, when post-processing the logged data, allow the data to be correlated with associated times and depths. Such clocks allow individual measurements performed during LWD to be assigned specific depths. One pre-condition for assuring accurate time (and thus depth) assignments is that both downhole and uphole clocks run synchronized.
One drawback associated with the use of LWD tools is that the downhole clock is typically subject to great temperature variations. Such temperature variations may occur in the range of, for example, 20 degrees Celcius to 175 degrees Celsius.
It is very difficult to provide a clock or other time measurement device that is accurate over such a large temperature range. Thus, in typical LWD processes, synchronization between the uphole and downhole clocks may be compromised due to inaccuracies in the downhole clock. This results in misalignment of log features recorded during the LWD process.